1. Field of the Invention
The present invention relates in general to reducing nitrogen oxide (NO.sub.x) levels in flue gas, and, in particular to a new and useful method and system for converting NO.sub.x to nitrogen gas (N.sub.2) through a combined heat exchanger and ammonia injection process.
2. Description of Related Art
In the power plant field, a flue gas stream is formed during the combustion of fuels such as coal, oil, natural gas, petroleum coke, etc., which are burned by electric power generating plants and many other industrial processes.
In these fields, it is common to use a selective catalytic reduction (SCR) reactor for removing NO.sub.x from the flue gas. The NO.sub.x removal process involves introducing an ammonia reagent into the flue gas for use in the SCR. There are several known methods used to remove NO.sub.x from the flue gas in SCR reactors.
A first known method uses anhydrous ammonia in order to reduce NO.sub.x levels. With a relatively small amount of energy, the anhydrous ammonia can be evaporated with either an electric source or with steam coils. The vaporized ammonia is then diluted with air in order to provide an adequate mass necessary to distribute the ammonia reagent evenly over a large ductwork cross-section. In this method, the diluted ammonia and air mixture is delivered to a grid of injection pipes located in the flue gas ductwork and upstream of a SCR catalyst bed. The injection pipes span the width of the flue gas duct and are closed at one end. The ammonia and air mixture is injected into the flue gas through nozzles or orifices that are sufficiently spaced along the injection pipes in order to provide an even distribution and thorough mixing of the ammonia with the flue gas. Major disadvantages associated with using this method include the safety concerns and precautions pertaining to the handling and storage of the anhydrous ammonia. Especially in highly populated areas, local government regulations often require that aqueous ammonia be used instead of anhydrous ammonia.
A second method for reducing NO.sub.x levels is to use an aqueous ammonia with an external heat source in order to evaporate the aqueous ammonia. The aqueous ammonia used is typically purchased in industrial grade form and is approximately 30% by weight ammonia and 70% by weight water. A dedicated heater, usually an electric-type heater, is used to heat dilution air to a level which is adequate enough to vaporize the ammonia and water mixture. A vaporization chamber or static mixer is the medium in which the phase change occurs. Usually, atomization air is required to assist in the break-up of the aqueous ammonia in order for fine droplets of the aqueous ammonia to enter the vaporization chamber. After vaporization, the ammonia and water air mixture exits the vaporization chamber and is delivered to an injection grid for injection into the flue gas as described above.
A major disadvantage associated with this method is that there is a parasitic power demand caused by the dilution air heater. A typical installation can have heater power demands ranging in the hundreds of KW range. Furthermore, there is great cost associated with this method due to the capital cost of the air heater and associated controls and hardware. Additionally, there are several maintenance problems associated with this method, particularly, burned-out heating elements which lead to costly maintenance down time.
A third method is to use an aqueous ammonia with the flue gas as the dilution and heating medium. This method comprises taking a hot slip stream of the flue gas from the ductwork, upstream of the SCR reactor, and in turn sending it through a vaporization chamber or static mixer. As described in the second known method above, the aqueous ammonia is injected into the vaporization chamber with atomization air in order to facilitate the phase change. As previously described, the ammonia-water-flue gas mixture exits the vaporization chamber and is delivered to an injection grid.
The major disadvantages associated with this method include the costly need for ductwork and insulation and the limited application for this method. This method is limited to "clean" flue gas which has nearly no dust, ash or sulfur oxides. Flue gas containing dust and ash is certain to clog or plug the small injection orifices in the injection pipes. Additionally, sulfur oxides form ammonium sulfate and bisulfate salts which cake on the spray nozzle in the vaporization chamber and plug the injection orifices of the injection pipes.
A fourth known method for reducing NO.sub.x in a flue gas is to spray aqueous ammonia directly into the flue gas upstream of the SCR catalyst bed. In this method, the aqueous solution is sprayed into the flue gas upstream of the catalyst bed in a manner similar to the way reagent is introduced into a selective non-catalytic reduction process (SNCR) in which a liquid ammonia derivative is sprayed in boiler high temperature regions in order to accomplish NO.sub.x reduction. The energy from the flue gas is used to accomplish the phase change.
A major problem associated with this method is that great residence time is required in order to vaporize the water and ammonia. Additionally, there is insufficient distance upstream of the catalyst bed for placing the injection pipes. This is further complicated by the requirement of protecting the SCR catalyst from liquid water in order to avoid contamination.
Through this method, the need to provide carrier mass is not met which means that the number of total nozzles in the cross-section of the flue gas is limited. Thus, this method limits the capability to have a uniform injection distribution. Furthermore, because the injection pipes are hot, the phase change occurs within the injection pipe before the reagent reaches the nozzle. This further inhibits the effort to achieve a uniform ammonia distribution.